There are many types of infrared thermal imaging systems in the prior art. The most widely used classes of infrared imagers employ photon detection and thermal detection. In the latter category, i.e. thermal detectors, the pyro-electric effect present in certain materials is employed as the primary detection means. While the pyro-electric capabilities of materials are often very good, and can achieve a noise equivalent temperature differences of less than 0.01 K, few detectors can approach that performance and still remain both commercially and practically usable. The main problems lie in electrical and thermal isolation of the sensor, electrical contacts to the readout, and the limiting of noise bandwidth which, together, all act to degrade the detector's response in an assembly or array of detectors.
As an example, many thermal detectors employ thin liquid crystals as the heat sensing medium. It is particularly difficult to temperature stabilize the liquid crystal media, as the electrical contacts thereto act as heat sinks and tend to dissipate the accumulated energy, thus decreasing the sensitivity of the system.
Others have attempted to avoid the heat dissipation problem by employing liquid crystal detection units in a light transmission arrangement. In specific, an infrared image is focussed on a liquid crystal detector which, in response to local temperature variations in the crystal medium, accordingly alters its local index of refraction. Subsequently, a polarized light beam is transmitted through the liquid crystal medium which interacts with the polarized light to locally alter the angle of polarization in accordance with the local changes in index of refraction. These changes are detected after the beam exits from the liquid crystal and enable the image to be reproduced. Transmission detection systems require reasonably thick liquid cells which exhibit both low thermal efficiency and significant crystalline noise. Such systems are described by B. F. Lamouroux et al. in "Signal-to-Noise Ratio Analysis of a Digital Polarimeter Application to Thermal Imaging", Review of Scientific Instruments, 54 (5), May 1983 pages 582-585; in "Infrared Video Camera at 10 Microns", Andre et al. Applied Optics Vol. 18, No. 15, Aug. 1979 pages 2607-2608, and in British published patent application GB 2 150 387 A, to Elliot et al. entitled "Thermal Imager".
In U.S. Pat. No. 4,160,907 to Bly, an infrared imager is described wherein a thin semiconducting film, such as vitreous selenium is employed as the infrared detector. This detector exhibits a temperature-dependent optical absorption spectrum which results in local areas on the detector varying in transmissivity in relation to radiation incident thereon. Detection of the infrared radiation is accomplished by transmitting light through the detector and sensing, on the other side thereof, local changes in the intensity of the transmitted light as an indication of the infrared image. This systems suffers a decrease in sensitivity due to the loss of brightness which occurs as the result of having to sense the light, after its passage through the absorption medium.
Accordingly, it is an object of this invention to provide a pyro-optic detector wherein the use of electrical contacts to the pyro-optic element are avoided.
It is another object of this invention to provide a pyro-optic detector wherein the detection element exhibits high thermal efficiency.
It is still another object of this invention to provide a pyro-optic detector which employs a detection material exhibiting a high temperature coefficient of refractive index.
Yet another object of this invention is to provide a pyro-optic detector which avoids losses inherent in transmission imaging systems.